1. Field of the Invention
The invention is in the field of imaging and more specifically in the field of ultrasonic imaging.
2. Prior Art
Ultrasonic imaging is a method of analysis used for examining a wide range of materials. The method is especially common in medicine because of its relatively non-invasive nature, low cost, and fast response times. Typically, ultrasonic imaging is accomplished by generating and directing an ultrasound beam into a material under investigation in a transmit phase and observing reflections generated at the boundaries of dissimilar materials in a receive phase. For example, in medical applications observed reflections are generated at boundaries between a patient's tissues. The observed reflections are converted to electrical signals (channel data) by receiving devices (transducers) and processed, using methods known in the art, to determine the locations of echo sources. The resulting data is displayed using a display device such as a monitor.
The prior art processes of producing an ultrasound beam and analyzing resulting echoes is called “beam forming.” The production process optionally includes defining “transmit” beam characteristics through aperture apodization, steering, and/or focusing. The analysis process optionally includes calculating a “receive beam” wherein received echoes are processed to isolate those echoes generated along a narrow region. This calculation includes the identifying one-dimensional line along which echoes are assumed to have been generated, and is therefore referred to herein as “echo line calculation.” Through beam forming a one-dimensional set of echolocation data is generated using each transmit and/or receive beam. Echolocation data is positional data relating to the physical location of one or more echo source and optionally includes intensity, velocity and/or similar physical information. Echolocation data may include post-beam forming raw data, detected data, or image data. Multidimensional echolocation data, such as an ultrasound image, is generated by scanning a field of view within the material under investigation using multiple transmit and/or receive beams.
The ultrasound beam transmitted into the material under investigation during the transmit phase is generated by applying electronic signals to a transducer. The ultrasound beam may be scattered, resonated, attenuated, and/or reflected as it propagates through the material under investigation. A portion of the reflected signals are received at transducers and detected as echoes. The receiving transducers convert the echo signals to electronic signals and optionally furnish them to an echo line calculator (beam former) that performs the echo line calculation inherent to analysis using a receive beam.
After beam forming, an image scan converter uses the calculated echolocation data to generate image data. In prior art systems the image formation rate (the frame rate) is limited by at least the total pulse return times of all ultrasound beams used to generate each image. The pulse return time is the time between the transmission of the ultrasound beam into the material under investigation and the detection of the last resulting reflected echoes. The limited frame rate may result in temporal artifacts caused by relative movement between the ultrasound system and a material under investigation.
FIG. 1 shows a prior art ultrasound system, generally designated 100. Ultrasound system 100 includes an element array 105 of transducer elements 110, a backing material 120, an optional matching layer 130, a transmit/receive switch 140 and a beam transmitter 150. Backing material 120 is designed to support element array 105 and dampen any ultrasound energy that propagates toward backing material 120. Matching layer 130 transfers ultrasound energy from transducer elements 110 into the material under investigation (not shown). Transducer elements 110, include individual transducer elements 110A-110H individually coupled by conductors 115 and 117, through transmit/receive switch 140, to a beam transmitter 150. Transmit/receive switch 140 may include a multiplexer 145 that allows the number of conductors 117 to be smaller than the number of conductors 115. In the transmit phase, beam transmitter 150 generates electronic pulses that are coupled through transmit/receive switch 140, applied to some or all of transducer elements 110A-110H, and converted to ultrasound pulses 160. Taken together, ultrasound pulses 160 form an ultrasound beam 170 that probes the material under investigation.
Ultrasound beam 170 may be focused to limit the region in which echoes are generated. When echo sources are restricted to a narrow region the calculation of echo location data may be simplified by assuming that the echo sources lie along a “transmit line.” With this assumption, the task of the echo beam calculator is reduced to a problem of determining the position of an echo source in one dimension. This position is established using the return time of the echo. The accuracy of this assumption and the spacing of transmit lines are significant factors in determining the resolution of prior art ultrasound systems. Finely focused beams facilitate higher resolution than poorly focused beams. Analogous assumptions and consequences are found in analyses involving calculated receive beams.
FIG. 2 shows a prior art focusing system in which element array 105 is a phased array configured to focus ultrasound beam 170 by varying the timing of electronic pulses 210 applied to transducer elements 110A-110H. In this system, electronic pulses 210, are generated at beam transmitter 150 and passed through transmit/receive switch 140. Electronic pulses 210 are delayed using a delay generator (not shown) and coupled to transducer elements 110A-H. Ultrasound beam 170 is formed when transducer elements 110A-H convert properly delayed electronic pulses 210 to ultrasound pulses 160 (FIG. 1). Once formed, ultrasound beam 170 is directed along a transmit beam line 250 including a focal point 230 with a resulting beam waist 240 characterized by a width of ultrasound beam 170. In a similar manner phased excitation of element array 105 is used to direct (steer) ultrasound beam 170 in specific directions. The cross-sectional intensity of ultrasound beam 170 is typically Gaussian around a focal point and includes a maximum along transmit beam line 250. The shape of ultrasound beam 170 may depend on aperture apodization.
In a scanning process, ultrasound system 100 sends a series of distinct ultrasound beam 170 along another, different transmit beam line 250 to form an image over more than one spatial dimension. A specific ultrasound beam 170 is optionally transmitted in several transmit/receive cycles before generating another ultrasound beam 170. Between each transmit phase a receive phase occurs, during which echoes are detected. Since each ultrasound beam 170, included in an ultrasound scan, requires at least one transmit/receive cycle the scanning processes may take many times the pulse return time. This pulse return time, determined by the speed of sound in the material under investigation, is a primary limitation on the rate at which prior art ultrasound images can be generated. In addition, undesirable temporal anomalies can be generated if transducer elements 110A-110H move relative to the material under investigation during the scanning process.
FIGS. 3A through 3E show a prior art scanning process in a phased array 310 of eight transducer elements, designated 110A through 110H. Subsets 320A-320E of the eight transducer elements 100A-110H are each used to generate one of distinct ultrasound beams 170A-170E. For example, FIG. 3A shows ultrasound beam 170A formed by subset 320A, including transducer elements 110A-110D. The next step in the scanning process includes forming ultrasound beam 170B using subset 320B including transducer elements 110B-110E as shown in FIG. 3B. In this example, a transmit beam line 250B associated with ultrasound beam 170B passes through a focal point 230B, which is displaced from a focal point 230A by a distance typically equal to the width of one transducer element 110. As shown by FIGS. 3C through 3E, each subset 320C through 320E, used to produce each ultrasound beam 170C through 170E, is displaced by one transducer element 110 relative to subsets 320B through 320D, respectively. Echoes detected in the receive phase, occurring between each transmit phase, are used to generate echolocation data and these echolocation data are typically combined to form an image suitable for display. The scan process may be repeated to produce multiple images.
In practice, phased array 310 may include sixty-four, one hundred and twenty-eight, or more transducer elements 110. The resolution of the echolocation data depends on the aperture and the number of transducer element 110, and on the degree to which transmit beam line 250 accurately represents possible echo sources within ultrasound beam 170. Representation of ultrasound beam 170A-E using beam line 250A-E is an approximation that determines the resolution of resulting echolocation data. A poor approximation will limit the resolution of the resulting echolocation data. A maximum width of ultrasound beam 170A-E is, therefore, limited by the desired resolution of the echolocation data. The accuracy of the approximation is a function of distance from focal points 230A-E, the approximation being less accurate at further distances.
Common practice includes generating several ultrasound beams with different focal point 230A-E, and using each set of received echoes to generate data near focal points 230A-E. Prior art data generation may be limited to an area near focal points 230A-E because, at further distances, the transmit beam line 250 approximation may not be sufficiently accurate to provide the echolocation data of a desired resolution. Typically one receive or transmit beam line 250 is generated for each transmit/receive cycle. The number of beams required to image an area is dependent on both the width and depth of the area to be imaged as well as the desired resolution. By using only echoes near focal point 230, only a small portion (e.g. <10%) of the total received signal is used, with the remainder of the received signal being discarded. The prior art makes inefficient use of detected signal. Similar disadvantages occur in systems utilizing synthetic receive lines.
In the prior art the area to be covered, transmit beam width, number of transmit beam 170, and echolocation data resolution are interdependent. The transmit beam width determines the minimum lateral resolution width of the echolocation data. Since each transmit beam 170 covers only a limited area, a greater number of transmit beam 170 are required to image a larger area. Use of a greater number of transmit beam 170 lengthens the minimum time required to generate an image.
Disadvantages of the prior art, such as an image formation rate restricted by pulse return time and inefficient signal use, have prevented prior art ultrasound systems from taking full advantage of advances in micro-processing power. The prior art endures these disadvantages in order to generate images with the highest possible resolution.